It is now well known to design real-time computer image generation (CIG) systems to provide realistic reproduction of images for a variety of simulator systems, such as flight simulators and the like. In such systems, image generation can be broken down into three separate processing stages: controller, geometry processor and display processor. Each of these three processing stages works, independently of the other, on data corresponding to one of three consecutive images to be displayed. The controller processes data in an image or scene for a fixed time and then passes its processed data to the geometry processor. The geometry processor has an equal amount of time to do its calculations and at the end of that time sends its results to the display processor. The display processor processes the data for final display on a video display. For each time interval, all three processors are kept busy, so that at any given time, the processors are working on three separate scenes. Data flows through the three stages in a sequential manner, so that to maximize processor efficiency, an image or scene is divided into a plurality of spans which are sequentially processed. Each span is, conceptually, a small (generally rectangular) area on the display screen, and is comprised of a matrix of picture elements (pixels). An object-to-be-displayed is comprised of at least one individual face, where each face is bounded by straight lines and is thus an uninterrupted, continuous area. A face, therefore, may cover a number of spans, and a span may contain a number of faces. The overall goal of an area (span) processing system is to maximize the efficiency of the three separate processors, and provide a realistic correct image while satisfying real-time processing constraints. It is now well known to reduce the total calculations and calculation time in situations where a plurality of objects were positioned in front of, or behind, each other, by utilizing a depth buffer to determine for each of a plurality of faces, forming the totality of object portions in each span, which face portion is in front of, and therefore is visible over, other face portions further from the observer. One depth-buffer-priority processing scheme is described and claimed in U.S. Pat. No. 4,825,391, issued Apr. 25, 1989 to the assignee of the present application, and incorporated herein in its entirety by reference. This reference is concerned with formation of a raster image, as on a normal CRT display, where the raster traces through the array of spans of an image.
It is also well known that an image can be formed upon a calligraphic-raster display device which not only allows the normal raster image to be displayed in a desired scanned fashion (e.g. left-to-right in each row, with a plurality of rows being sequentially written from top to bottom of the screen), and also allows calligraphic point features to be individually placed upon the screen. Typically, the raster image can be overwritten by each calligraphic point feature, as the writing electron beam is deflected in vector fashion so as to draw lines and points over the raster image. This overwriting procedure produces clear sharp points and lines free from distracting effects caused by rasterizing the image, for example, scintillations, stairstepping and the like phenomena are reduced, if not completely removed. However, proper display of a total scene requires that the occultation of different object polygons of the raster image not only be determined, as by the depth (Z) buffer method of the aforementioned Patent, but also requires determination of the proper occultation relationships between the raster and the calligraphically-displayed point features (which are not necessarily single "points", but may have a real, non-zero radius), and in fact, between different ones of the calligraphically-displayed point features themselves. For example, if a calligraphic point feature is visually located behind the surface of the polygon in the raster image, that calligraphic point feature should not be displayed; of course, since the point feature has not only a center location but also a radial extent (i.e. is not a single "point"), the point feature can partly occult, or be partly occulted by, a polygon surface in the raster image and only a part of the circular point feature would then be visible and be displayed in the resulting total image. Similarly, because the calligraphic features can have different radial extent, even if different point features have centers lying at the same point, proper occulation between different calligraphic point features must also be obtained. Accordingly, it is therefore highly desirable to provide a single method for resolving priority between calligraphic-displayed point features and both other calligraphic-displayed point features and raster-displayed features; it is highly desirable that this process utilize the depth-buffer-based method previously used for resolving priority between plural raster display features.